1. Field of the Invention
This invention relates generally to devices for mixing gaseous fuel and air in the correct proportions for combustion in a gas engine, and is particularly concerned with a venturi mixing device for replacing the spring mass, diaphragm actuator of existing gaseous fuel carburetors for natural gas engines and the like, and with a method of retrofitting a carburetor with such a device.
2. Related Art
The majority of gaseous fuel carburetors for internal combustion engines have an air/fuel mixing section which is comprised of a spring mass system. This has the potential to create unstable engine performance due to interaction with the upstream pressure regulator or emissions control valve. The carburetor was originally designed to run open loop with a mechanical upstream pressure regulator, and on applications that did not have strict emissions requirements. In recent years emissions requirements have become very strict, and more sophisticated upstream air/fuel ratio controls have either replaced the mechanical pressure regulator entirely or supplement its operation. These devices are typically electromechanical and respond to the commands of a computer controller using a closed loop control strategy, based on the output of an oxygen sensor, located in the exhaust stream of the engine. These new systems can respond very quickly and often create unstable interactions between the carburetor and the upstream fuel valve. This interaction is possible because the air/fuel mixing section of the carburetor is comprised of a dynamic spring mass system
There are several existing types of gaseous fuel carburetors and the majority use a diaphragm to actuate the air fuel mixing section against the force of a spring. The smallest model eliminates the diaphragm and uses a piston as the operator. The diaphragm is typically an elastomer coated fabric and is susceptible to chemical and thermal degradation, rupture due to engine backfiring, abrasive damage, and the like. The air/fuel mixing section of the carburetor also has sliding surfaces that are prone to wear. Down time of an engine due to required maintenance in an industrial application, such as a pump on an oil pipeline, can result in the loss of thousands of dollars from lost production.
Another deficiency of known gaseous fuel carburetors is the limited ability to thoroughly mix the air and fuel together. The fuel is introduced into the center of the air stream from a single location. This can create a lean mixture at the outer edges of the flow stream and a richer mixture in the center. This makes it more difficult for the engine to completely burn all incoming fuel, which often results in higher exhaust emissions and a reduction in horsepower.
The limited number of existing gaseous fuel carburetor models and sizes may require the user to install a unit that is too large for the application. A carburetor excessively large for an engine may cause starting troubles. If certain application parameters change significantly, such as fuel composition, the user may be required to change to a different model of carburetor and modify the associated plumbing and linkage of the engine.
Venturi mixers have been developed for gaseous-fueled engines in order to overcome some of the problems of the existing gaseous fuel carburetors with a spring-mass system forming the air/fuel mixing section. One example is the FMV6 Mixing Venturi manufactured by Continental Controls Corporation of San Diego, Calif. Other such devices are manufactured by Woodward and Heinzmann. These mixers are not installed in the existing carburetor. Instead, they replace the carburetor entirely, which often creates substantial installation difficulties and costs. The physical envelope of these mixers is typically very different from the existing carburetor. This requires major modifications to the engine's air intake system, which could result in thousands of dollars of lost production time. Another major deficiency is that these mixers do not include a butterfly for controlling the air/fuel mixture to the engine. In some instances the existing butterfly is separable from the carburetor and can be used with custom adapters. If the butterfly is not separable from the carburetor, a new butterfly may have to be purchased. If the location of the butterfly is different from the previous installation, additional changes to the governor linkage will be required. An additional mixture screw will also be required. Conventional retrofit difficulties increase significantly when the engine uses more than one carburetor, which is often the case.
Therefore, what is needed is a system and method that overcomes these significant problems found in the conventional systems as described above.